Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through various samples, such as membranes, films, envelopes, bottles, packages, containers, etc. (hereinafter collectively referenced as “test films” for convenience). Typical test films are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terepthalate (PET), polyvinylidene chloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second chambers. The first chamber (commonly referenced as the driving or analyte chamber) is filled with a gas containing a known concentration of the target analyte (commonly referenced as a driving gas). The second chamber (commonly referenced as the sensing chamber) is flushed with an inert gas (commonly referenced as a carrier gas) to remove any target analyte from the cell. A sensor for the target analyte is placed in fluid communication with the sensing chamber for detecting the presence of target analyte that has migrated into the sensing chamber from the driving chamber through the test film. Exemplary permeation instruments for measuring the transmission rate of oxygen (O2), carbon dioxide (CO2) and water vapor (H2O) through test films are commercially available from Mocon, Inc. of Minneapolis, Minn. under the designations OXTRAN, PERMATRAN-C and PERMATRAN-W, respectively.
Permeation testing instruments employ a very low mass flow through rate through the instrument to limit the creation of any pressure differentials in the instrument that could impact humidification of the test and/or carrier gases or create a pressure-induced driving force across a test film. This low mass flow rate through the instrument imposes a significant time delay between measurements from different testing cells as the feed line to the sensor is flushed with the carrier gas from the sensing chamber of the newly selected testing cell.
A substantial need exists for a permeation instrument capable of contemporaneously measuring target-analyte transmission rates from a plurality of testing cells with minimal changeover time between measurements from different testing cells.